The braking system of a truck operates pneumatically. Both the truck (tractor unit) and the towed vehicle (semi-trailer or trailer) are braked and held stationary by means of air. The air pressure is generated in the air supply section and, via the foot brake pedal, metered and passed on to the air-consuming section. In the air-consuming section are the brake chambers, which, via a mechanism, press the brake linings against the brake disc or brake drum. When stationary, the parking brake is activated. The air is vented from the system and the powerful springs in the spring brake chambers operate the rear brakes. To release the brakes again, air pressure is sent from the parking brake valve to the spring brake chambers to compress the spring again. The brakes are then released.
The pages about the basic principles of air pressure brakes, the air supply, service and auxiliary/parking brake, and the towed vehicle brake are described according to the conventional braking system. In this we still find many components such as an ALR, which, with the advent of EBS, we no longer see as a separate component under the vehicle.
Modern trucks and trailers use the Electronic Braking System (EBS), in which the brake pedal position is measured by sensors and sent to the EBS control unit. Via CAN bus, the desired deceleration is sent to the modulators. The front axle, rear axle and trailer modulators control the brake pressure to the brake chambers, depending on the load. The EBS system has many advantages over the conventional braking system: due to the electric control, a high deceleration is possible with little pedal force, and better harmonisation between truck and towed vehicle can also be achieved.